28 
ON THE GEOLOGY AND NATURAL HISTORY 
Ascended Bear peak, which is a solitary upheaval in the plain near the north fork of 
the Shyenne, and seems to be cut off from the main portion of the Black hills by a valley 
about six miles in width. The strata which underlie the surface of this valley are more or 
less disturbed. Bear peak is composed entirely of trap rocks, no granite being seen, and 
is evidently a protrusion or outburst of igneous rocks. Much of the rock is very porous 
trachyte; portions of it are more compact, breaking into irregular fragments, producing a 
peculiar ringing under the blows of a hammer like clinkstone. This rock also has a 
gritty structure, sometimes a dark gray or bluish gray color. Immense quantities of 
broken rocks cover the sides of the peak to the depth of twenty or thirty feet; the strata 
are vertical, or nearly so, and the ridges of upheaved Cretaceous and Jurassic beds which 
surround the peak in nearly a circular form, gives to the whole a crateriform appearance. 
The lowest unchanged rock exposed by this upheaval is the blue limestone bed E of the 
vertical section, then, alternate beds of red and yellowish gray argillaceous grits, shales, 
and sandstones are revealed, containing great quantities of Jurassic fossils. From the 
summit of the peak at a distance of twenty or thirty miles towards the north, the Deer’s 
Ears and Slave butte are visible, and though the country in the vicinity is underlaid by 
Cretaceous formation No. 4, these hills are, I am confident from their appearance, com¬ 
posed of the sands and clays of the Lignite Tertiary, monuments left to indicate that the 
Tertiary beds once covered the surface of the country in that region. From the valley 
on the south side of Bear peak we ascend to the Black hills by a series of step-like ridges, 
composed of a full series of the Cretaceous, Jurassic, Carboniferous, Silurian, and meta- 
morphic strata, with here and there protrusions or outbursts of igneous rocks. In one 
locality basaltic columns similar to those forming Stone peak were observed, some lying in 
nearly a horizontal position, others dipping at an angle of 20° to 40°. These columns are 
five-sided, the sides varying from eight to twenty inches in width. The rock is quite hard 
and compact in its structure, not differing from that which enters into the formation of 
Stone peak. 
Near Beaver Creek, formation No. 2 is very largely developed, presenting its usual litho¬ 
logical characters, and containing great quantities of fragmentary fish remains, but no 
other fossils. No. 1 in this vicinity is composed of variegated clays, grits, and sandstones, 
with indistinct vegetable impressions, fossil wood, and a few uncharacteristic saurian bones. 
No. 3 is also exposed by the upheaval of the beds, containing its usual fossils, but possess¬ 
ing the character of a laminated calcareous sandstone, instead of the soft homogeneous cal¬ 
careous marl of the Missouri river. 
